mfd: cpcap: Implement IRQ sense helper

CPCAP can sense if IRQ is currently set or not. This
functionality is required for a few subdevices, such
as the power button and usb phy modules.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>